The Case for a Thermal Comfort Rating Metric

February 14, 2017
Spring 2017
A version of this article appears in the Spring 2017 issue of Home Energy Magazine.
Click here to read more articles about Passive House Approach

Energy efficiency has been a driving force in the home-building industry for decades. In recent years, the industry has increasingly adopted rating methods to measure a home’s energy efficiency. RESNET has produced the HERS Index, and DOE has produced the Home Energy Score (HES). Both systems have demonstrated value as user-friendly metrics to predict and assess the energy performance of new and existing homes. In today’s market, stakeholders are also recognizing that home performance extends beyond energy efficiency, and are seeking new ways to assess that performance. For example, DOE is teaming with Lawrence Berkeley National Laboratory to develop a metric for indoor air quality performance. Several groups, including RESNET, are also focused on implementing a residential water efficiency rating. As ways to broaden residential performance ratings gain momentum, there is room for a metric that rates a home’s ability to deliver thermal comfort. After all, a home is expected to be comfortable, and heating, cooling, and hot water are major drivers of both comfort and home energy use.

Ari Rapport
Ari Rapport is a program manager at IBACOS. He is a certified HERS Rater and has over ten years’ experience performing home energy ratings.

In the past, evaluating a home’s thermal comfort performance has been tied directly to the performance of the HVAC system, and much of the responsibility for that performance has been placed squarely (and often unfairly) on the HVAC system designer and installer. While the HVAC system plays a significant role in delivering comfort in a home, there are many other factors with significant impacts on a home’s comfort performance: layout of the home, orientation and solar exposure, lighting, insulation levels and quality of installation, air movement, thermal mass, and so on. All of these factors, and others, work together to create an environment in the home that is capable of providing a range of comfort conditions. This is a complex dynamic even before you consider the occupants of the home, which adds a whole other layer of subjectivity and variability!

Thermal comfort is subjective. It varies from person to person; it depends on one’s metabolism, activity level, and clothing; it can be affected by one’s health and by any number of other personal factors. It also can be influenced by factors that are established by the design of the home itself. These include solar heat gains, the capacity and layout of the heating-and-cooling system, and the R-value of the insulation. Occupants perceive thermal comfort based upon the surrounding indoor air temperature, air currents, humidity levels, and other quantifiable factors, including the change in thermal conditions over time and from one room to another (see Figures 1 and 2). ASHRAE’s Standard 55 Thermal Environmental Conditions for Human Occupancy is a well-researched document that defines the range of factors that influence human comfort, and is an excellent resource for understanding the complex interactions of both the human (subjective) and the design and material (objective) factors that influence comfort in the home. Despite the subjective aspects of thermal comfort, home buyers want to be sure that their home will deliver the comfort they desire.

Room to Room Thermal Conditions

Room to Room Thermal Conditions
FIgure 1. Occupants perceive thermal comfort based upon the surrounding indoor air temperature, air currents, humidity levels, and other quantifiable factors, including the change in thermal conditions over time.

Change in Thermal Conditions Over Time

Change in Thermal Conditions Over Time
Figure 2. Changes in thermal conditions over time and from one room to another also influence occupant comfort.

Professionals can use the HERS Index or the HES as tools to help design, build, and retrofit homes to be energy efficient. The complex calculations and energy models behind the HERS Index and HES have been “boiled up” to provide builders and home performance contractors with easy-to-understand scores to discuss energy use with homeowners and prospective buyers. A similar calculation and modeling process with a simplified score for thermal comfort would give these professionals a common language for discussing thermal comfort with those same clients.

IBACOS research and feedback from the field indicates that even as homes evolve to use less energy, they are not necessarily becoming more comfortable. Ironically, homes with the most complex and state-of-the-art HVAC systems can still be uncomfortable due to less obvious reasons such as too much solar gain, poor air mixing in the home, draftiness, or simply improper commissioning of the HVAC system after installation. At this time, the housing industry has no simple way to give thermal comfort a voice. While ASHRAE’s Standard 55 provides all the right information and insight to help improve this situation, it currently remains too complex and inaccessible for the typical builder, contractor, or homeowner to apply its wisdom usefully. We need easy-to-use tools to adequately discuss comfort, to rate it, and to examine it in the context of other home performance aspects such as energy use and indoor air quality. A gap exists. The time has come to bridge that gap and develop a tool to define the ability of homes to provide needed comfort. Enter the Thermal Comfort Rating Metric (TCRM).

Consumer Demand

Today’s homeowners are becoming ever more savvy as they seek to optimize value in their lives. Home buyers are more demanding and seek feedback from their devices (including their cars, phones, bank accounts, and more) on real-time performance, and expect a higher level of personalization and control. In the era of the “Internet of Things” and the “connected home,” we are beginning to expect more direct interaction with our homes, including greater feedback and more personalized options for controlling the systems within our homes. In the complexity of system interactions within a home, it is important to distill this complexity into simple choices for consumers to make, so they feel empowered to make decisions that affect the performance of their homes in personalized ways. As professionals in housing, we must develop a tool that both responds to the consumer, yet is based in sound comfort science so that we can maximize performance and minimize risk. A TCRM could become that tool, to enable consumers to make decisions regarding their home’s comfort in meaningful ways, and to empower them with the knowledge to trade off comfort with other home performance aspects such as energy efficiency, indoor air quality, cost, and aesthetics. At the time of sale of a new home, or before a major home retrofit, a TCRM tool could help consumers make informed choices about options, and help them feel confident that the home they buy can deliver the comfort they expect.

Housing Industry Trends

Codes, standards, and voluntary programs are driving greater energy efficiency in residential new construction. For example, the International Energy Conservation Code (2015 IECC) is becoming increasingly stringent as it evolves, and has adopted an Energy Rating Index (ERI) performance path that uses a numerical scale like the HERS Index to verify compliance with the code. Even state and local residential incentive programs are placing more emphasis on energy efficiency, and standards such as ASHRAE 90.2-2007 are helping to pave the way toward greater energy efficiency in homes. Architects, builders, and developers are adopting Passive House and DOE Zero Energy Ready Home standards as they try to differentiate themselves in the marketplace. As codes, standards, and voluntary programs continue to drive greater energy efficiency, the industry needs effective mechanisms to quantify other performance aspects like comfort. A TCRM could provide that effective mechanism.

Building Science Technical Translation

The building industry is rich with information, research, and experience related to thermal comfort for human occupancy. ASHRAE has developed a “Thermal Comfort Tool” for calculating thermal comfort parameters and making thermal comfort predictions using thermal comfort models defined in Standard 55. And, most building scientists consider comfort an essential aspect of good building design and construction. However, most stakeholders in the residential sector are not building scientists or engineers. They may not fully understand, or know how to apply, existing comfort standards, guidelines, and tools in a useful way to improve the performance of their homes. The available comfort tools and information must therefore be made accessible in a way that these stakeholders can use it to accurately define, predict, measure, and assess the ability of a home to deliver thermal comfort. A TCRM could become a useful mechanism to harness the best comfort science and distill it into a more accessible and usable format for the industry. Ideally, a TCRM could build off existing energy models used to calculate the HERS Index or HES, and with a few additional inputs could produce a score that reflects a home’s comfort performance. The exact mechanism for developing a TCRM needs to be determined, but it should integrate with other home performance metrics to the extent possible, and must be robust, standardized, repeatable, and easy to understand and apply

The Business Case

Stakeholders in the housing industry—designers, builders, trades, raters, real estate agents, developers, and retrofit contractors, among others—are deeply invested in the satisfaction of their customers, and those customers want their homes to be comfortable. When customer satisfaction is high, business is better. Comfort is an aspect of quality that a builder can manage if it has the right tools to do so. High-quality homes sell more readily and for higher prices; they enhance a stakeholder’s market reputation; and they make a home more pleasant to live in. Poor comfort, on the other hand, can be a liability to business and unpleasant for a homeowner. A TCRM will give all of these stakeholders a simplified way to convey the value of their product in terms of comfort.

Builders must also manage risk; this is an absolute necessity for their businesses. Builders who fail to effectively manage risk spend a lot of money trying to keep their customers happy, through either repeated callbacks or expensive litigation proceedings. Often, homes that are marketed as “high performance” because they achieve a certified energy efficiency designation (like Energy Star for New Homes) may not actually deliver high performance in areas like comfort, indoor health, and durability. This is a real problem for builders who market and sell homes as “high performance” but in fact they are not. As a result, they are not properly setting the expectations for their customers. Even worse, builders may actually think they are delivering the performance they are not, and are inadvertently creating risk for themselves and their customers. Many of these risks—and the resulting opportunities—associated with high-performance homes have been captured by the DOE in its concept of a “building science tipping point.”

learn more

Get more info on the ASHRAE Thermal Comfort Tool

Learn more about DOE’s concept of a “building science tipping point."

The bottom line is that builders need accurate and useful tools to help them define achievable performance targets for their homes and meet these targets, better manage customer expectations related to the performance of their homes, and deliver on their performance promises to better manage their risks. A TCRM will help the housing industry shift its focus to overall performance, and specifically to thermal comfort.

A TCRM can help raise the “value stakes” for thermal comfort. No tool can promise perfect results all of the time, but a TCRM will help define achievable parameters and help set expectations for all stakeholders. It can help builders and manufacturers to sell their products and improve their bottom lines. It can help homebuyers to get their home to deliver the thermal comfort that they expect. It can open pathways to the innovation of new business models and new products, technologies, and processes to improve the comfort performance of homes while meeting energy efficiency targets set by code, government, and other programs.

IBACOS received funding from the U.S. Department of Energy’s Building America program to validate the need for a TCRM.

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